cc1: Report an error for multiple actions unless separated by -main-file-name (#91140)

When multiple actions are specified, the last one is used and others are
overridden. This might lead to confusion if the user is used to driver's
`-S -emit-llvm` behavior.

```
%clang_cc1 -S -emit-llvm a.c     # -S is overridden
%clang_cc1 -emit-llvm -S a.c     # -emit-llvm is overridden
%clang_cc1 -fsyntax-only -S a.c  # -fsyntax-only is overridden
```

However, we want to continue supporting overriding the driver action
with -Xclang:

* `clang -c -Xclang -ast-dump a.c` (`%clang -cc1 -emit-obj ...
-main-file-name a.c ... -ast-dump`)
* `clang -c -xc++ -Xclang -emit-module stl.modulemap`

As an exception, we allow -ast-dump* options to be composed together
(e.g. `-ast-dump -ast-dump-lookups` in AST/ast-dump-lookups.cpp).

Author: MaskRay

clang/include/clang/Basic/DiagnosticFrontendKinds.td

@@ -134,6 +134,8 @@ def err_fe_no_pch_in_dir : Error<
     "no suitable precompiled header file found in directory '%0'">;
 def err_fe_action_not_available : Error<
     "action %0 not compiled in">;
+def err_fe_invalid_multiple_actions : Error<
+    "'%0' action ignored; '%1' action specified previously">;
 def err_fe_invalid_alignment : Error<
     "invalid value '%1' in '%0'; alignment must be a power of 2">;
 def err_fe_invalid_exception_model

clang/lib/Frontend/CompilerInvocation.cpp

@@ -2841,6 +2841,30 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
     }
 
     Opts.ProgramAction = *ProgramAction;
+
+    // Catch common mistakes when multiple actions are specified for cc1 (e.g.
+    // -S -emit-llvm means -emit-llvm while -emit-llvm -S means -S). However, to
+    // support driver `-c -Xclang ACTION` (-cc1 -emit-llvm file -main-file-name
+    // X ACTION), we suppress the error when the two actions are separated by
+    // -main-file-name.
+    //
+    // As an exception, accept composable -ast-dump*.
+    if (!A->getSpelling().starts_with("-ast-dump")) {
+      const Arg *SavedAction = nullptr;
+      for (const Arg *AA :
+           Args.filtered(OPT_Action_Group, OPT_main_file_name)) {
+        if (AA->getOption().matches(OPT_main_file_name)) {
+          SavedAction = nullptr;
+        } else if (!SavedAction) {
+          SavedAction = AA;
+        } else {
+          if (!A->getOption().matches(OPT_ast_dump_EQ))
+            Diags.Report(diag::err_fe_invalid_multiple_actions)
+                << SavedAction->getSpelling() << A->getSpelling();
+          break;
+        }
+      }
+    }
   }
 
   if (const Arg* A = Args.getLastArg(OPT_plugin)) {

clang/test/Frontend/multiple-actions.c

@@ -0,0 +1,7 @@
+// RUN: not %clang_cc1 -S -emit-llvm -main-file-name %s 2>&1 | FileCheck %s --check-prefix=ERR1 --implicit-check-not=error:
+// ERR1: error: '-S' action ignored; '-emit-llvm' action specified previously
+
+// RUN: not %clang_cc1 -main-file-name %s -emit-llvm-only -emit-llvm -S 2>&1 | FileCheck %s --check-prefix=ERR2 --implicit-check-not=error:
+// ERR2: error: '-emit-llvm-only' action ignored; '-S' action specified previously
+
+// RUN: %clang_cc1 -S -main-file-name %s -emit-llvm -o /dev/null